Cemented Paste Backfill Consolidation with Deposition-Dependent Boundary Conditions

M. Shahsavari, M. Grabinsky

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Mining Geotechnics

ABSTRACT: ASTE BACKFILL CONSOLIDATION WITH DEPOSITION-DEPENDENT BOUNDARY CONDITIONS M. Shahsavari and M. Grabinsky University of Toronto, Toronto, Ontario, Canada ABSTRACT Cemented paste backfill (CPB) in underground mines is increasing in popularity due to its relatively high delivery rate, homogeneous as-placed properties, and tight filling characteristics as compared to other backfill methods. A significant engineering challenge is to determine the pressures acting on the backfill barricades, which in turn is dependent on the amount of dissipated pore water pressure in the CPB and thus its degree of consolidation. Gibson (1958) studied the one dimensional self-weight consolidation characteristics of a soil wherein its thickness increases with time. The applicability of the Gibson (1958) solution to backfill materials is studied by Fahey et al. (2010), who introduced lower and upper bound solutions with a varying coefficient of consolidation as deformation occurs. Both Gibson (1958) and Fahey et al. (2010) considered a zero pore water pressure head on the top of each layer when deposited. However, at the top of the actual deposited backfill is a slurry which imposes a finite pressure head and permeability. The effect of this boundary condition on the consolidation characteristics of CPB is studied in the proposed paper using Terzaghi's 1-D consolidation theory. This result is then generalized by modeling the consolidation process with modified boundary condition using FLAC3D. The time-dependent material properties, which are a function of stage of binder hydration, are calibrated using laboratory test results. RÉSUMÉ Le remblai en pâte cimentée (CPB) est de plus en plus utilisé dans les mines souterraines étant donné un taux d'approvisionnement relativement élevé, des propriétés en place homogènes ainsi qu'un remplissage complet comparativement à d'autres méthodes de remblayage. Un défi d'ingénierie important est de déterminer les pressions sur les barricades à remblai. Ces pressions dépendent de la quantité d'eau interstitielle dans le CPB et donc, de son degré de consolidation. Gibson (1958) a étudié les caractéristiques unidimensionnelles de consolidation d'un sol sous son propre poids lorsque l'épaisseur du sol augmente en fonction du temps. Fahey et al. (2010) ont étudié l'applicabilité de la méthode de Gibson (1958) à des matériaux de remblayage en introduisant des limites inférieure et supérieure ainsi qu'un coefficient de consolidation variable en fonction de la déformation. Gibson (1958) et Fahey et al. (2010) ont considéré une pression d'eau interstitielle nulle au-dessus de chaque couche de sol déposée. Cependant, la partie supérieure du remblai est composée d'une boue, ce qui impose une charge hydraulique et une perméabilité à ce matériau. L'effet de cette condition limite sur les caractéristiques de consolidation de CPB est étudié dans cet article en utilisant la théorie de consolidation 1-D de Terzaghi. Le résultat est ensuite généré en modélisant le processus de consolidation avec une condition limite modifiée à l'aide du logiciel FLAC3D. Les propriétés du matériau, qui sont variables en fonction du temps et dépendent également de la phase d'hydratation du liant, sont calibrées en utilisant des résultats d'essais de laboratoire. 1 INTRODUCTION It is crucial in underground mining operations to fill the voids (stopes) after the extraction of the ore. One of the backfilling materials is cemented paste backfill (CPB). CPB has gained popularity over the past 15 years due to its high delivery rate and strength compared to other backfilling materials. One of the key issues during and after backfilling is how much stress is being applied to the barricade. The barricade stresses are induced from the development of effective vertical and hence horizontal stresses in the backfilling material, as well as pore water pressures. If cement hydration is ignored, the main cause of development of effective stresses is due to the consolidation of CPB under its own weight with time. Gibson (1958) considered the problem of the consolidation of a clay layer where its thickness is increasing with time. He derived an analytical solution for the problem considering mostly the same assumptions as Terzaghi (Terzaghi, 1943) considered for the solution of the problem of consolidation. Fahey et al. (2010) applied the Gibson solution to the consolidation of CPB. They tried to address the limitations of the Gibson solution to mine backfilling. It was concluded that a varying coefficient of consolidation (cv) can have an effect on the pore pressure generation and thus must be considered in the cases where the initial analysis using the Gibson solution with a constant cv suggest high degrees of consolidation at short times. However, due to strength gain and at the same time decrease of the permeability coefficient the assumption of constant cv can be justified for a preliminary analysis as was also discussed by Fahey et al (2010). Thus for the sake of simplicity of the analysis the effect of varying cv is ignored in the present study. Previous studies on the simulation of the consolidation of CPB considered a zero pore pressure boundary conditions on top of the layer (Fahey et al, 2010; Helinski, 2008; Li and Aubertin, 2009). However, field monitoring

RÉSUMÉ: TED PASTE BACKFILL CONSOLIDATION WITH DEPOSITION-DEPENDENT BOUNDARY CONDITIONS M. Shahsavari and M. Grabinsky University of Toronto, Toronto, Ontario, Canada ABSTRACT Cemented paste backfill (CPB) in underground mines is increasing in popularity due to its relatively high delivery rate, homogeneous as-placed properties, and tight filling characteristics as compared to other backfill methods. A significant engineering challenge is to determine the pressures acting on the backfill barricades, which in turn is dependent on the amount of dissipated pore water pressure in the CPB and thus its degree of consolidation. Gibson (1958) studied the one dimensional self-weight consolidation characteristics of a soil wherein its thickness increases with time. The applicability of the Gibson (1958) solution to backfill materials is studied by Fahey et al. (2010), who introduced lower and upper bound solutions with a varying coefficient of consolidation as deformation occurs. Both Gibson (1958) and Fahey et al. (2010) considered a zero pore water pressure head on the top of each layer when deposited. However, at the top of the actual deposited backfill is a slurry which imposes a finite pressure head and permeability. The effect of this boundary condition on the consolidation characteristics of CPB is studied in the proposed paper using Terzaghi's 1-D consolidation theory. This result is then generalized by modeling the consolidation process with modified boundary condition using FLAC3D. The time-dependent material properties, which are a function of stage of binder hydration, are calibrated using laboratory test results. RÉSUMÉ

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Cite this article:
M. Shahsavari; M. Grabinsky (2014) Cemented Paste Backfill Consolidation with Deposition-Dependent Boundary Conditions in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper422,author = M. Shahsavari; M. Grabinsky,title = Cemented Paste Backfill Consolidation with Deposition-Dependent Boundary Conditions,year = 2014}